The present invention relates in general to a lock-up mechanism for mounting a flexible printing plate on a plate cylinder of a printing press.
In printing presses such as a conventional web fed rotary printing press, a printing plate is wrapped about a plate cylinder and is mounted under tension on the plate cylinder by clamping the opposite ends of the printing plate between inner walls of an axial slot formed on the periphery of the plate cylinder and a plate clamping shaft disposed in the slot. The plate clamping shaft is then rotated. After clamping, the printing plate should be uniformally tensioned in all directions, and the clamp is secured in place during printing to prevent movement of the printing plate and resulting registration errors.
In a conventional plate clamping device of this type, an eccentric plate clamping shaft is journaled by bearers at both longitudinal ends of the plate cylinders, and the opposite ends of the printing plate are inserted into gaps between the plate clamping shaft and the inner walls of the slot. As a result, the printing plate is clamped by the wedge action of the plate clamping shaft caused by the tension of the printing plate. As a consequence, the long plate clamping shaft journaled at its opposite ends has a tendency to be bent by the tension of the printing plate so that the printing plate is not under uniform tension.
In printing presses such as offset lithographic printing presses the use of thin, flexible plates is typical and are basically planographic, that is, without relief, and are treated so that the surfaces have areas which attract or repel oil and water to create areas of image differentiation. To mount such plates on the surface of the press cylinder, it is essential to insure that the plate conforms closely to the outer surface of the plate cylinder. Mounting is usually effected by forming a longitudinal slot in the plate cylinder and inserting thereinto one of a variety of biasing or clamping devices called lock-ups. Often the trailing edge of the plate is bent back on itself so that the bent portion can fit over the edge formed in the cylinder by the longitudinal slot and then be held in position by some form of spring or clamp. Alternatively, mechanisms have been provided in which the plate leading edge is rigidly clamped.
In other prior art devices, the plate leading edge is held between two parts which are of an approximately trapezoidal shape. The two trapezoidally shaped parts are forced inwardly to clamp the end of the plate within a slot. The trailing edge of the plate is held between a sloped surface of one of the trapezoidal parts and a similarly sloped wall of the slot in the plate cylinder. Numerous other types of lock-up devices are known in the prior art.
Disadvantages of the prior art is that many of the lock-up devices require mechanisms external to their ends and the plate cylinder for securing the lock-up device in position. Also, many lock-up devices are known to loosen during operation of the printing press because they do not have a set position for locking the leading and trailing edges of the printing plate.